WASHINGTON (CNN) -- The United States on Friday sought clarification from Iran in response to its counter-offer involving shipping low-enriched uranium for refining abroad, senior administration officials said.

The apparent Iranian refusal of a nuclear proposal centers on Iran not wanting to send its uranium to Russia, as stated in a document agreed on earlier this month at the headquarters of the International Atomic Energy Agency (IAEA) in Vienna, Austria.

Iranian negotiators had tentatively agreed to the proposal last week.

Tehran is proposing instead that its uranium be enriched on Iranian soil by a third country, under IAEA supervision, Obama administration officials said. Another possibility suggested by Iran is to ship out the uranium in several shipments, not in one bulk stockpile.

The officials said the Iranians also want to have talks with the United States about playing a role to ensure the technical safety of the reactor, presumably a way to draw the White House into bilateral talks.

The officials did not regard Iran's counter-offer as formal because it was delivered orally by the Iranian delegate in Vienna.

Speaking in Pakistan, U.S. Secretary of State Hillary Clinton said leading powers that have been involved in the negotiations -- Britain, China, France, Germany, Russia and the United States -- should refrain from immediately judging the status of nuclear talks.